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Using the 10-metre Keck Observatory telescopes, astronomers have probed one of the most compact dust discs ever resolved around another star.

The study revolves around the Keck Interferometer Nuller (KIN), which provides high resolution measurements by combining the incoming light from the two Keck telescopes and blocking unwanted light from the star. “This is the first compact disc detected by the KIN, and a demonstration of its ability to detect dust clouds a hundred times smaller than a conventional telescope can see,” says Christopher Star of NASA’s Goddard Space Flight Center.

This diagram compares 51 Ophiuchi and its dust discs to the Sun, planets and zodiacal dust in the Solar System. Zones with larger dust grains are red; those with smaller grains are blue. Planet sizes are not to scale. Image: NASA/GSFC/Marc Kuchner and Francis Reddy.

The dust discs reside around a young hot B-type star known as 51 Ophiuchi, and may represent a rare, nearby example of planet formation, although there is not yet any evidence to show if planets have formed yet.

The new data answered a mystery about what made 51 Ophiuchi's dust disc appear so compact while its spectra suggested that the dust orbited at much larger distances. The answer was simply that the star had two debris discs. A compact inner disc extends roughly four astronomical units and is accompanied by an outer disc that extends hundreds of times farther. If picked up and transported to our own Solar System the inner disc would almost reach Jupiter, and the outer disc would extend from Saturn's orbit to ten times the distance of the Kuiper belt.

“Our new observations suggest 51 Ophiuchi is a beautiful protoplanetary system with a cloud of dust from comets and asteroids extremely close to its parent star,” says team member Marc Kuchner.

Dust in our own Solar System typically forms inside Jupiter's orbit, as comets break up near the Sun and asteroids collide within the Asteroid Belt. This dust reflects sunlight and sometimes can be seen as a wedge-shaped sky glow called the zodiacal light before sunrise or after sunset. Dusty discs around other stars that arise through the same processes are called 'exozodiacal' clouds.

“Our study shows that 51 Ophiuchi’s disc is more than 100,000 times denser than the zodiacal dust in the Solar System,” says Stark. ”This suggests that the system is still relatively young, with many colliding bodies producing vast amounts of dust.”

KIN observations were combined with data already collected by the Spitzer Space Telescope and the European Southern Observatory’s Very Large Telescope Interferometer in Chile to probe the structure of the dust disc further. The astronomers found that the inner disc contains fine particles with sizes of 10 micrometers and larger which the outer disc contains much smaller grains, just one percent the size of those in the inner disc, similar in size to the particles in smoke. The combined observations also revealed that the outer disc appears more puffed up, extending farther away from its orbital plane than the inner disc.

“We suspect that the inner disc gives rise to the outer disc,” says Kuchner. This process occurs by the dust-making collisions of asteroids and comets. Large dust particles naturally spiral toward the star, but the opposing pressure of the star's light pushes smaller particles out of the system. This process occurs in our own Solar System but is thought to operate much more efficiently at 51 Ophiuchi, which is 260 times more luminous than the Sun.

The Nuller instrument will be used in the future to help understand how and when these asteroid belts form and how dust from the star’s debris disc might interfere with direct imaging of planets orbiting another star.

The findings appear in the 1 October issue of The Astrophysical Journal.

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